Incidence: Ten to 20 persons a year infected in rural areas of Western United States. Globally, 1,000 to 3,000 cases a year.

Treatment: Antibiotics. If not treated promptly can cause death. About 14 percent of all U.S. plague cases are fatal.

Place in history: Millions of Europeans died in the Middle Ages when flea-infested rats inhabited homes and workplaces.

Source: Centers for Disease Control and Prevention

NEW YORK (CNN) --A New Mexico couple who traveled to New York have been hospitalized with what is believed to be the first case of bubonic plague in the city in a century, said health officials.

The couple arrived in the city last Friday and went to the hospital two days later with high fever and swollen lymph nodes. The man, 53, is in critical condition and on life support at a Manhattan hospital; his 47-year-old wife is in stable condition, said officials. Both are in isolation at the hospital.

"Today, we are announcing what are likely to be the first cases of bubonic plague in New York City in 100 years," said Dr. Thomas Frieden, the health commissioner of New York City.

Tests on the man were "presumptive positive" for the plague, and his wife is suffering from similar symptoms, with tests pending, he said. It could be as long as 48 hours before health officials get official confirmation that the two have the plague, he said.

Frieden cautioned New Yorkers not to be alarmed by the news. Bubonic plague "does not spread from person to person," he said.

"There is no risk to New Yorkers from the two individuals who are being evaluated for plague," Frieden said. "These patients became ill within 48 hours of arriving in New York City. Therefore, we are confident that their exposure occurred in New Mexico."

More than half of the plague cases in the United States are in New Mexico, Frieden said. A wood rat and fleas from the rodent that were found on the couple's property in Santa Fe, New Mexico, tested positive in July for plague, Frieden said.

Bubonic plague is a bacterial disease in rodents transmitted to humans through the bites of infected fleas. Pneumonic plague, a more serious form of the disease, occurs when plague bacteria are inhaled after direct contact with infected animals, including rodents, wildlife and pets.